Honestly, you've got a lot of options here. The easiest and most obvious, at least to me, is to have William II stick around longer. As long as he loves even somewhat longer, William II will probably produce an heir, which means that Constance isn't his legal heir, which means that the Holy Roman Empire doesn't have a claim on Sicily.
Going back earlier, you could always have the Battle of Dyrrachium result in Alexios I getting killed - OTL, he suffered a wound to his forehead, and thus it isn't inconceivable that he could be killed. Weakening Constantinople opens southern Greece up for Norman expansion. You could also always have some sort of Sicilian-Genoese, Sicilian-Venetian, or Sicilian-Pisan war (or something similar) that allows for Sicilian expansion in northern Italy. Or, you could have the Sicilians move into Hungary and Croatia. Really, with a POD early enough in Sicilian history, you could see the Sicilians move into just about any area in the central Mediterranean.